1. Field of the Invention
The field of this invention relates to corneal implants to correct refractive errors in a patient. More particularly, it relates to a corneal ring inlay device for reversibly altering the curvature of the central cornea without intruding on the central cornea, which is particularly useful in correcting vision defects.
2. Description of the Background Art
Deviations from the normal shape of the corneal surface or the axial length of the eye produce errors of refraction in the visual process. The eye in a state of rest, without accommodation, focuses the image of distant objects exactly on the retina. Myopia is that refractive condition in which, with accommodation completely relaxed, parallel rays are brought to focus in front of the retina. One condition which commonly causes myopia is when the length of the eye is greater than normal. In this condition, rays converge or focus in front of the retina in the vitreous of the eye. When the rays reach the retina, they have diverged, forming a circle of diffusion and consequently a blurred image. A concave lens is needed to correct the focus of the eye for myopia.
The classical treatment of these forms of refractive error of the eye is with the use of eyeglasses or contact lenses, both of which have well-known disadvantages to the user. Recently, surgical keratorefractive techniques have been developed to change the refractive condition of the eye in order to correct both hyperopic and myopic errors. The results of such surgical procedures are not entirely satisfactory. One such technique, referred to as keratomileusis, was introduced by Barraquer of Columbia in 1961. This technique involves grinding of a corneal button into an appropriate shape to correct myopia or hyperopia. It is often unsatisfactory, in part because it often induces additional refractive errors to the eye. These refractive errors are not compensatible, in part because suturing can cause radial asymmetry of the cornea that subsequently results in astigmatic error, and in part because suturing causes scarring of the corneal tissue, which generates scar tissue that is not transparent to light.
In a related surgical technique, keratophakia, a donor cornea is ground into a convex lens and inserted in the lamella in order to correct aphakic hypermetropia. Radial keratotomy was introduced in modern times (1972) by Fyodorov of the USSR to correct astigmatism and myopia. Correction of astigmatism by asymmetrically altering the corneal curvatures is prone to producing aberrant refractive corrections, in part because of suturing and in part because of remodeling of the altered stroma. Such surgery does not have a fully predictable outcome, and furthermore any non-spherical flattening of the cornea on healing results in an eyesight defect that cannot be corrected by lenses.
It is thus seen that present procedures in keratorefractive surgery are best limited to conditions where other more standard corrective practices are found ineffective. It is generally recognized that the limiting factor in such surgical techniques is the complexity involved, not only with multiple incisions in corneal tissue necessary for accomplishing such procedures, but also complex suturing patterns, resulting in a gross restructuring of the eye.
It is thus clear that there is an important need for alternate methods of achieving a change in anterior corneal curvature without extensive surgical intervention.
U.S. Pat. Nos. 4,452,235, 4,671,276, 4,766,895 and 4,961,744 to Kera Vision, Inc. disclose attempts to meet this need with a surgical apparatus consisting of a flexible plastic, split end, flat adjusting ring that is inserted into the stroma of the cornea of the eye. A metal dissecting ring, held in a circular shape by a special holder, is connected to one end of the adjusting ring in order to part the stroma and provide a pathway for the adjusting ring as the ring is rotated through the stroma. The ends of the adjusting ring are moved to change the curvature of the cornea to a desired shape in accordance with the desired visual correction, after which the ends of the adjusting ring are hooked to each other in order to maintain the desired shape; the dissecting head is then detached from the adjusting ring and removed from the eye. Due to the resulting stress on the cornea, such rings may erode through the cornea, resulting in severe damage.
In oral presentations at a January 1988 meeting of CLAO and at a May 1989 meeting of ARVO, E. Barraquer described flat, intrastromal rings for correction of myopia consisting of thin sections cut from silicon tubing. Ring were inserted by dissecting a large intrastromal pocket, and then inserting the flexible silicone ring into the pocket. The surface of the cornea was cut only at the point of the initial incision; Bowman's membrane was not cut in a circular fashion.
Zhivotovskii, D. S., USSR patent no. 388746, describes an alloplastic, flat, geometrically regular, annular ring for intracorneal implantation of an inside diameter that does not exceed the diameter of the pupil. Refractive correction is accomplished primarily by making the radius of curvature of the surface of the ring larger than the radius of curvature of the surface of a recipient's cornea in order to achieve flattening of the central area of the cornea. Surgical procedures for inserting the rings are not described.
Krasnov, M. M., Ann. Ophthalmol., 24:165 (1992) describes a machined homoplastic annular transplant to be grafted on top of the cornea concentrically with the visual axis in order to correct myopia.
The advantages of a ring system for altering the curvature of the cornea, but one without the complicated apparatus and surgical and erosion problems attendant upon the use of the aforementioned split end apparatus, are encompassed in the present invention which is disclosed and claimed below.